canvas: Make 2D context state creation failable and use dom_canvas_backend pref for backend selection (#38310)

Before script just crashed in those cases because IPCSender was dropped,
now we send `None` to tell script about the failure and fail getContext
or registerPainter accordingly.
This PR also unifies `dom_canvas_{backends}_enabled` prefs into
`dom_canvas_backend` which is more flexible in multi-backends scenarios.

Reviewable per commit.

Testing: Added servo specific WPT test.

---------

Signed-off-by: sagudev <16504129+sagudev@users.noreply.github.com>

tests/wpt/vello_canvas_subsuite.json

@@ -1,7 +1,7 @@
 {
   "vello_canvas": {
      "config": {
-        "binary_args": ["--pref", "dom_canvas_vello_enabled"]
+        "binary_args": ["--pref", "dom_canvas_backend=vello"]
      },
      "include": ["/html/canvas/element"]
   }